In many applications, it is necessary to quickly change the output current of a current source. Ideally, the current would instantly change between two values. In a practical circuit, the current will exhibit undue settling delay; it will transition most of the way to its new value fairly quickly but take much longer to completely settle to the final value. This slowly settling error term is typically due to a reference transistor being disturbed by the switching. For example, after switching between output currents, the final current through a reference MOSFET in a current mirror will be delayed due to the time required to charge the relatively large gate capacitance of the output MOSFET. Many examples of switched current sources have been described in the prior art.
FIG. 1 shows one prior art method for switching between two current levels. MOSFETs M1 and M2 are connected as a current mirror, where the current through MOSFET M2 is controlled by the current through MOSFET M1. MOSFET M2 will typically be much larger (have a wider gate) than MOSFET M1, so the currents will be proportional to their respective sizes. The current source I1 is permanently connected to the drain of MOSFET M1, and the current source I2 is selectively connected to the drain via a switch 10. When clock signal PHI is high, the switch 10 is closed and the input of the M1/M2 current mirror is I1+I2. When PHI is low, the current drops to I1. As the drain current of MOSFET M1 changes, its gate to source voltage will be slow to react since MOSFET M2 is usually much larger than MOSFET M1 and has a large gate capacitance. A slowly changing gate voltage on the M1/M2 current source results in slow settling of IOUT through MOSFET M2.
FIGS. 2 and 3 show slightly faster methods for switching current, since the switching by the clock signal PHI has less effect on the gate voltage of the current mirror reference MOSFET M3 (FIG. 2) and MOSFET M7 (FIG. 3).
In FIGS. 2 and 3, the input to MOSFET M3 (FIG. 2) and MOSFET M7 (FIG. 3) stays constant, and the current mirror output is split into two branches: one that is always on and one that is completely switched on or off. MOSFETs M3 and M7 will be referred to as reference MOSFETs. The “always on” branches include MOSFET M4 in FIG. 2 and MOSFETs M9 and M10 in FIG. 3. The switched branches include MOSFETs M5 and M6 in FIG. 2 and MOSFETs M11 and M12 in FIG. 3.
In the circuits of FIGS. 2 and 3, the switching transient at the gates of the reference MOSFETs M3 and M7 is smaller than in FIG. 1, but there will still be a disturbance coupled through the gate capacitance of MOSFETs M5 and M11.
A further disadvantage of these circuits is that they require extra voltage headroom since there is a second MOSFET in series with the output MOSFET M5 and M12.
What is needed is a current source that can more quickly and accurately change its current output.